This invention relates to a bowl liner for a conventional truck of a railroad car. A railroad car is mounted on a truck assembly using a center plate assembly which is rotatably held in a bowl-shaped crater in the truck, familiarly referred to as the "bowl". The center plate assembly comprises a right-cylindrical center plate (vertical side wall and planar bottom in the horizontal plane). Since the center plate and bowl are each typically formed from cast steel, it is evident that direct contact of the center plate in the bowl would produce abrasive wear which would be unacceptable. To minimize the wear it has been conventional to interpose a metal wear plate between the center plate and the bowl with the expectation that the wear plate will be sacrificed in due course when it is replaced. A manganese steel liner is typically used between the center plate assembly on the car, and the bowl of the truck' bolster (see Car and Locomotive Cyclopedia, 1974 Edition at pages 513-25), the hardness of the liner being tailored to provide the wear resistance desired. To extend the expected useful life of the wear plate which is commonly made from a low-friction alloy, the wear plate is lubricated. In this scheme of operating a railroad car one can expect to re-lubricate a wear plate every 2-3 months, assuming the car travels, on average, 100,000 miles in a year.
To avoid the problem of re-lubricating the bowl, various liners of synthetic resinous material have been proposed with the avowed purpose of overcoming the cost of maintaining the bowl and center plate assembly as well as to avoid the use of lubricant which gets ejected and distributed on the rail tracks and the ground thereunder, when the railroad car is in motion.
As long as eighteen years ago, in U.S. Pat. No. 3,944,298 to Cannon, it was proposed to keep the conventional vertical metal split-ring wear liner in the bowl, but to substitute a "flat disc of plastic material" for the conventional metal horizontal wear liner. Use of the metal vertical wear liner required lubrication which deleteriously affected the "plastic material". Further, even without fragmentation, when normal wear of the metal liner occurred, grit was lodged between the center plate and the bowl liner, damaging the liner. The "plastic material" is identified therein as being "low friction, semi-flexible, high-load carrying ability" (see col 2, lines 45-46), and commercially available under the Pennlon.RTM. brand name, sold by Dixon Corporation. Cannon believed that by using only the horizontal wear liner, the problems associated with movement of a unitary bowl liner would be minimized, if not obviated. They were not, as evidenced by the failure of the substitute Pennlon.RTM. wear liner, known to be high molecular weight polyolefin (believed to be polyethylene) to provide satisfactory service.
Recognizing the promise of high molecular weight polyolefin material, about sixteen years ago, Chierici et al disclosed the use of a bowl liner of ultrahigh molecular weight (UHMW) high density polyethylene (PE) in U.S. Pat. No. 4,075,951. Despite the long period over which worrisome details of such bowl liners could have been ironed out they have not been satisfactory, mainly because they were unduly sensitive to thermal degradation and to compressive deformation. It was assumed that the polyolefin would be adequately insensitive to the relative movement of a unitary bowl liner, relative to both the center plate and the bowl, which movement, in a rail car carrying a normal load for which it is designed, generates a large abrasive force. There is no indication in the art as to what the response of a polyolefin bowl liner was to the abrasive action of grit.
It is generally acknowledged that the deficiency of the Pennlon.RTM. and UHMW PE bowl liners was that neither was sufficiently stable under conditions of elevated temperature and pressure, and that they failed because of the peculiar resilient and elastomeric characteristics of the polyolefin which rendered it pliable. Further, with relative movement of a unitary bowl liner at elevated temperature in the range above 43.3.degree. C. (110.degree. F.), prior art bowl liners are so sensitive to compressive deformation that the useful life of a prior art "plastic" bowl liner in a rail car is less than that of a conventionally lubricated metal wear plate and vertical wear liner.
It is more critical that the polymer matrix of a satisfactory bowl liner be substantially rigid, than it is that there be low sliding friction between the center plate and the bowl. By "substantially rigid" is meant that the polymeric liner used herein, when subjected to a distortion force normally encountered within the environment of a bolster's center bowl at ambient temperature, is capable of resisting the distortion force applied to the liner as it is oriented in the bowl, and capable of maintaining the liner's formational shape thereafter. Further, a material which is substantially rigid is not pliable, that is, not bendable or shapeable without being damaged, after the shaped material is removed from a RIM machine.
Unlike the prior art which sought to substitute a pliable plastic material for the conventional metal wear liner, it was decided to test a polymer matrix which emulated, to as large an extent as practical, the physical properties of the metal wear liner. Thus, the polymer matrix of the liner used herein is non-extensible in the temperature range from 25.degree. C. to about 100.degree. C., therefore non-pliable and essentially non-deformable, and cannot have a 300% modulus which characterizes an elastomer. The term "elastomer" is used herein in its accepted meaning to refer to "a polymeric material such as a synthetic rubber or plastic, which at room temperature can be stretched under low stress to at least twice its original length and upon immediate release of the stress returns with force to its approximate original length (McGraw Hill Dictionary of Scientific and Technical Terms, pg 648, 5th Edition, McGraw Hill Book Co.).
High friction forces are known to generate temperatures in the range from 110.degree. F.-250.degree. F. at which compressive deformation of the liner is instrumental in the derailment of cars, particularly 125-ton articulated double stack rail cars. Therefore, routinely, one skilled in the art seeks to minimize the sliding friction by manipulating such properties as the material of the wear plate, or bowl liner, the surface condition of each of the components of the center plate and bowl assembly, the contamination of those surfaces by foreign matter, and the type of lubricant, if one is used. Clearly, one cannot expect to control the ambient temperature and it is unrealistic to seek to operate a car with a load much smaller than the load it is designed to carry.